This invention relates to the valve art, and more particularly, to ball valves. The invention is particularly applicable to a new and improved ball valve and seat assembly for a valve of the type having a so-called "floating ball" and, more particularly, to a valve wherein the floating ball is end-loaded or top-loaded, and will be described with particular reference thereto. However, it will become readily apparent to those skilled in the art that the invention is capable of broader applications and could be adapted for use in other types and styles of valves.
Ball valve constructions in commercial use typically employ annular seats or seat rings formed of resilient and/or deformable plastic for sealing engagement with the ball. A pair of such seat rings are positioned adjacent inlet and outlet openings. The ball itself is mounted for a slight amount of free movement or shifting axially of the seat when the ball is in a valve closed position under fluid pressure conditions. Such shifting causes the ball to act against and flex the downstream seat ring to enhance its sealing engagement with the ball. The amount of such flexing varies in accordance with the fluid pressure involved.
At conditions where the upstream seat seals against the ball member at a valve closed position, a problem occurs with a "blowing-in" bulge on the upstream seat at the first opening of the valve. As the valve is being opened, the upstream seat must momentarily span the opening or fluid passage extending through the ball and hold back fluid pressure. During this short period of time until the upstream seat seal is relieved by further opening of the valve, the portion of the seat spanning the fluid passage can deform into the passage under fluid pressure. With a small opening in the ball, the seat is quite rigid when loaded as a beam in bending and can easily bridge the gap. As the valve size and ball opening increase, the section modulus of the seat does not proportionately increase to retain the same stiffness. Thus, the seat may deflect and deform further into the ball opening to form a bulge in that sector of the seat ring.
As repetitive recycling occurs throughout the life of the valve, the bulge in the upstream seat becomes more pronounced and actually operates to cam a floating ball against the downstream seat as the ball member is rotated to the valve open position. The resulting camming action further stresses, even cutting, the downstream seat, and may ultimately distort and wear the seat into a non-operative condition. The bulge on the inlet seat cams the ball off center and holds it away from both seats while closed causing seat leakage in the closed position.
Where soft plastic seats are employed, such as those made of polytetrafluoroethylene, particular problems encountered with cold flow creep in the seat at non-contained portions aggravate wear and undesirable seat deformation. A sealing upstream seat which has a generally unsupported front face can creep in toward the center of the valve when the valve is closed and under a long duration static fluid pressure drop across the upstream seat. Untimately, as the seat creeps forward it can cave-in entirely, or before that point it typically curls into the orifice of the ball such that when the valve is operated the seat is torn. This problem becomes more pronounced as the valve size increases.
The downstream seat can also creep in toward the center of the valve by its generally unsupported annular front face. When the valve is in the closed position, the seat may be displaced upstream by the ball member pushing into it under pressure load.
Where large ball members are employed in larger sized valves, the ball member weight additionally contributes to deformation and cold flow problems. As the seats deform after a period of use the ball member may sag to the bottom of the valve, thereby providing a clearance at the top of the valve for a leak path.
To provide improved compensation for seat ring wear, distortion and creep, as well as seat tolerance and ball sag, while providing an elastic support which effects a low pressure force bias, metal frusto-conical disc springs have been disposed in operative engagement with the seat ring rear face. The disc spring provides seat elasticity not dependent on the plastic memory of the plastic material. In addition, it is known to provide a support ring opposite the disc spring to contain the plastic seat ring. The support ring operates to provide additional seat ring support for bridging the ball passage at valve opening to minimize the blowing-in bulge and the associated reflected distortion on the downstream seat. Additionally, the support ring confines the seat to minimize cold flow creep distortion. An essential element of a valve design including a support ring is a support shoulder or counterbore opposite of the valve shoulder which provides a bearing or support surface for the support ring and ultimately the seat ring. Without such a support surface, the support ring is free to move away from the seat ring and precludes proper seat ring support.
In single-side end-loaded or top-loaded ball valves, although support rings are desirable, the structure of the valve body itself has precluded the formation of such a support ring support shoulder. In a single-side end-loaded ball valve the inclusion of such an element would block loading of the ball. In a top-loaded ball valve such a design would block loading of a seat ring assembly.
One suggestion for an item to support the seat ring in place of a shoulder-supported support ring has comprised a rigid casing that encases a portion of the flexible seat ring, generally about its radially outermost wall, and which further includes a flange normally depending therefrom and disposed in operative engagement with the seat ring front face. The flange limits cold flow creep and inhibits blowing-in of the seat ring. However, a particular problem with such a construction is that the casing need be formed about the perimeter of the seat and, accordingly, requires consumption of seat assembly area which could more usefully comprise flexible seat material. Another disadvantage of a casing support is that it limits the flexibility of virtually the entire plastic seat, including hindering the seat from moving forward against the ball. Thus, while the casing may provide support against seat creep and blowing-in bulge, it may actually hold back a seat and spring combination from compensating for wear, tolerance and ball sag. As the valve is cycled and the seats are worn, the spring may have to overcome both the elasticity of the seat ring and the rigid support of the casing to effect seat to ball engagement.
It has, therefore, been desired to develop a ball valve and seat assembly for a floating ball type of valve applicable to top-loaded and single side end-loaded ball valves in both the small and large sizes.
The present invention contemplates a new and improved construction which overcomes all of the above referred to problems and disadvantages and provides a new and improved floating type ball valve and seat assembly which will facilitate operation and sealing of the valve.